In the production of foundry mould parts, the binding agent systems which cold-harden with formation of polyurethane have achieved an increasing importance. These binding agent systems consist of two components, namely a polyol (normally dissolved in a solvent) having at least two OH-groups in the molecule and a polyisocyanate (usually also dissolved in a solvent) having at least two NCO-groups in the molecule. The two components, which are added separately to the mould material mixture, react in the mould material mixture to a hardened polyurethane binding agent, this reaction taking place in the presence of catalysts which assure a rapid reaction and thus satisfactorily short hardening time. As catalysts, apart from other substances such as metallo-organic compounds, tertiary amines in particular may be considered, which are introduced into the mould-making mould as lightly volatile amines with a carrier gas after the forming of the mould material mixture.
The polyol component is usually a condensation product, dissolved in a solvent, of (optionally substituted) phenols with aldehydes (hereinafter termed "phenolic resin" for short), which possesses a small to medium degree of condensation and comprises a fairly large number of free OH-groups in the molecule. In specific cases, especially in the case of mould parts for lower casting temperatures, the polyol component may, however, also be a solution of an oligomeric, dimeric or monomeric phenol body, for example of a terphenol, bisphenol or dihydroxybenzol. For all these polyols there is available a large number of (generally polar) solvents. The solutions are normally adjusted to a solids content of 40-60% by wt. and can contain also usual additives.
As polyisocyanate component, in principle any polyisocyanates having at least two NCO-groups in the molecule may be considered. Preferred are aromatic polyisocyanates, for which diphenyl methane-4,4'-diisocyanate, 2,2', 6,6'-tetramethyl diphenyl methane 4,4'-diisocyanate, diphenyl dimethyl methane-4,4'-diisocyanate and diphenyl-4,4'-diisocyanate may be named as typical examples. The polyisocyanates may form the polyisocyanate component in pure form or dissolved in an organic solvent (preferably a mixture of aromatic hydrocarbons having a boiling range above 150.degree. C.). In the case of a solution, the concentration of the polyisocyanate in general is above 70% by wt.
For the production of a mould material mixture, a granular base mould material (quartz sand, chromite sand, olivine sand, zirconium sand or the like) is mixed with the two binding agent components, the ratios of the two components being approximately in the range from 0.5 to 1.5 parts by wt. polyisocyanate component to 1 part by wt. polyol component and preferably of such values that a virtually stoichiometric ratio of the NCO-groups to the OH-groups results. The mould material mixture is then worked to form foundry mould parts by being placed in a mould-making mould, if necessary compacted and thereafter hardened, for example by brief gasification with a slightly volatile tertiary amine (such as dimethylethylamine, triethylamine or the like). The mould parts can then be removed from the mould-making mould.
Even as early as the gasification stage, the mould parts achieve a measurable strength ("initial strength"), which slowly increases after the end of gasification to the final strength values. In practice, sufficiently high initial strengths are desired for the mould parts to be removed from the mould-making mould if possible immediately after gasification and the mould-making mould if possible immediately after gasification and the mould-making mould can then be available again for a new moulding operation. At the same time, however, practice demands that the time during which the mould material mixture mixed with the two binding agent components can be stored before further processing to mould parts (the so-called "sand life") shall be as long as possible and that the initial strength values shall change as little as possible during this period.
The less the two binding agent components enter into a premature reaction with one another during storage of the mould material mixture, the better will the sand life be. Accordingly, it is possible to increase the sand life by addition of inhibitors of the urethane reaction; these inhibitors must be such in type and quantity that they only inhibit the spontaneous premature forming of urethane during storage of the mould material mixture, and not the desired forming of urethane after addition of the hardening catalyst, e.g. after amine gasification. It is known, for this purpose, to add to the polyisocyanate component of the binding agent a small quantity (for example up to 2% by wt.) of acid chlorides, both aliphatic and aromatic acid chlorides and acid sulphochlorides, such as acetyl chloride, stearyl chloride, benzoyl chloride, toluol sulphochloride or phthaloyl chloride being suitable. Our own, earlier West German patent application No. P 34 05 180.5 (now DE-05 3405 180) in West Germany and equivalent to U.S. Pat. No. 4,558,074 describes, for this purpose, an addition of up to 2% by wt. of chlorosilanes, such as for example trimethyl chlorosilane, dimethyl dichlorosilane, methyl trichlorosilane, tetrachlorosilane, diethychlorosilane, vinyltrichlorosilane and diphenyl dichlorosilane to the polyisocyanate component.
The known inhibitors lead, also, actually to an improvement in the sand life up to a doubling of same, therefore for example to an increase of the sand life from one hour to two hours. In many practical cases, however, this is still insufficient and in particular the known inhibitors are not sufficiently active when the mould material mixture is manufactured with regenerated old sands. Such regenerated old sands, which have increasing importance in foundry technology, usually have a temperature above 30.degree. C., with the result that the urethane reaction is accelerated, and they also contain iron oxide and other impurities having a catalytic effect on the urethane reaction, and therefore are specially critical in respect of the sand life.